Markers are substances which are used to tag products, typically petroleum products, for subsequent detection. The markers are normally dissolved in liquids to be identified, then subsequently detected by performing a simple physical or chemical test on the tagged liquids. Applications of markers include use by the government to ensure that the appropriate tax has been paid on particular fuels. Oil companies also mark their products to help identify those who have altered their products.
Markers have been used in petroleum products for many years. Many have proven extremely useful in providing definite means of identifying particular fuels for the purpose of criminal activity and theft. For example, U.S. Pat. No. 6,482,651 discloses a class of alkylated phenolphthalein esters that are particularly useful in this manner due to their ease of use and ease of testing. However, such marker compounds suffer a disadvantage in that they must be formulated with large amounts of co-solvents in order to, first, stabilize the concentrated solutions that are most desirable for markers of this type; and, second, to aid in the dispersion of the marker into the petroleum fluid to achieve the low dosage rate (<20 ppm) usually employed with fuel markers.
The co-solvents most often used are polar aprotic solvents. N-octylpyrolidinone, N-methylpyrolidinone, dimethylsulfoxide, dimethylformamide have proven especially well suited. The problem is one of cost. That is, these aprotic solvents are much more expensive than petroleum based hydrocarbon solvents. The aprotic solvent may be used at anywhere from 25 to 35 wt. % of the marker composition. Cresolphthalein dibutyrate, as taught in U.S. Pat. No. 6,482,651, requires at least 28 wt. % n-octylpyrolidinone. Otherwise, the phthalein precipitates on standing overnight in the freezer and will not disperse into diesel fuel even with vigorous stirring. One good test for solubility is to combine 10% by weight phthalein into kerosene. Kerosene is a very difficult solvent to work with. It has very non-polar characteristics and most dyes and markers have very poor solubility in pure kerosene.
In view of the above, it would be desirable to provide marker compounds which require less or zero co-solvents to achieve the desired stability and dispersion characteristics.